States Of Single-walled Carbon Nanotubes Research Articles

Near-infrared magneto-optical study of excitonic states in single-walled carbon nanotubes under ultra-high magnetic fields

Singlet excitonic states at the first subband-edge in single-walled carbon nanotubes (SWCNTs) have been studied through near-infrared magneto-absorption spectroscopy under magnetic fields to 105.9 T. Well-resolved absorption spectra of stretch-aligned SWCNT(CoMoCAT)-gelatin films were obtained above 100 T. By the application of magnetic fields in parallel to the alignment of SWCNTs, peak shift toward the lower energy was observed for (8, 4) and (7, 6) tubes and the opposite behavior was observed for (7, 5) and (6, 5) tubes. Above 28.8 T, new peaks emerged at the higher energy side of the peak for the (8, 4) and (7, 6) tubes, and at the lower energy side of the peaks for the (7, 5) and (6, 5) tubes. The magnetic splitting between the existing peak and the new peak was symmetric for every tube, which is in line with the energy splitting due to the Aharonov-Bohm effect. Judging from the energetic positions where the new peaks emerged, the singlet dark excitonic state locates at the lower energy than the singlet bright one in the (7, 5) and (6, 5) tubes while it is suggested strongly that the bright one locates at the lower energy in the (8, 4) and (7, 6) tubes.

Coherent phonons in excited-state carbon nanotubes: A simulation by tight-binding molecular dynamics

We have made tight-binding molecular dynamics simulations on the coherent phonons induced by electron–phonon coupling in the low energy optically excited states of single-walled carbon nanotubes (SWCNTs) with different diameters and chiralities. It is found that: (1) SWCNT’s radial breathing mode (RBM) and longitudinal optical (LO) mode are easily excited due to their direct coupling with band electrons, which then causes band gap oscillations with the same frequencies as those of RBM and LO modes. (2) Excitation of coherent phonons exhibits an interesting family-behavior. For example, for the family-II tubes, such as (11,0), (17,0) and (10,5) tubes with their mod( n− m,3) =2, the amplitude of their coherent RBM phonons in the first optical E 11 transition is smaller than that in the second optical E 22 one, while the opposite situation appears for a family-I tube with its mod( n−m,3) =1, e.g., (10,0) and (16,0) tubes. (3) Transverse optical (TO) modes cannot be excited in zigzag (10,0) and (11,0) tubes, but can be in a chiral (10,5) tube. The above obtained results are in good agreement with experimental observations.

(Invited) Probing Localized Excitons and Band Gap States in Single-Walled Carbon Nanotubes

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Tailoring Properties of Carbon Nanotube Dispersions and Nanocomposites Using Temperature-Responsive Copolymers of Pyrene-Modified Poly(N-cyclopropylacrylamide)

Despite their immense potential, the ability to control the dispersion and microstructure of carbon nanotubes remains a hurdle for their widespread use. Stimuli-responsive polymers show conformational changes with an applied external stimulus (pH, temperature, light, etc.). The dispersion of carbon nanotubes by thermoresponsive polymers is shown to enable the macroscopic properties of aqueous suspensions to be tailored as a function of temperature. This work presents the synthesis, characterization, and use of temperature-responsive poly(N-cyclopropylacrylamide) (PNCPA) polymers containing 1, 3, and 5 mol % pyrene-bearing repeat units to tailor the dispersion state of single-walled carbon nanotubes (SWNT) in water. Turbidity measurements show that the lower critical solution temperature (LCST) of PNCPA decreases with increasing pyrene content. Viscosity measurements on aqueous SWNT suspensions stabilized with pyrene-functionalized PNCPA show highly entangled and well-dispersed nanotube microstructure abov...

Exact diagonalization of Hubbard models for the optical properties of single-wall carbon nanotubes

Excitonic states of single-walled carbon nanotubes (SWNTs) have usually been calculated using many-body perturbation theories or mean field approaches because a large number of sites cannot be considered within an exact diagonalization (ED) calculation based on the Hubbard model. We use a small crystal approach and show that, for the π structure of nanotubes, an ED calculation is possible. We apply this approach to small-diameter SWNTs and the results show that a crossing of the first bright state with the second excited states occurs when U, the correlation parameter of the Hubbard model, equals 4t, where t is the hopping integral. Two or three strong two-photon absorption (TPA) states are found at energies above the first bright state for U/t⩽3. Beyond this value, these states become relevant for TPA below the first bright state. A number of dark states are always calculated below the first bright state at energies that, in the intermediate coupling regime, are of the order of tens to hundreds of meV. This result seems to be consistent with recent experiments.

Host–guest interactions in azafullerene (C59N)-single-wall carbon nanotube (SWCNT) peapod hybrid structures

The effect of azafullerene encapsulation on the electronic states of single-wall carbon nanotubes (SWCNTs) is investigated; UV-vis-NIR absorption and photoluminescence spectroscopy shows that the interaction between SWCNTs and the encapsulated azafullerenes is originated from the weak intermolecular forces, which suggests a lack of strong doping effect such as electron transfer between them.

A Near-Infrared Spectroscopy in Magnetic Fields Above 100 T

Development of the first measurement system for near-infrared absorption spectra between 0.9 and 1.7 μm under ultra-high magnetic fields is reported. Spectral signals were integrated for 1 μs with an InGaAs photodiode array around the top of a very short pulsed megagauss field. The magnetic fields were generated using a single-turn coil system. The measurement system was demonstrated in the study of exciton states in single-walled carbon nanotubes up to megagauss fields. A nearly noiseless absorption spectrum with well-resolved absorption peaks was obtained at 105.9 T in the Voigt configuration where the magnetic field was applied parallel to the alignment of the nanotubes.

Temperature Controlled Dispersion of Carbon Nanotubes in Water with Pyrene-Functionalized Poly(N-cyclopropylacrylamide)

Despite their immense potential, the ability to control the dispersion and microstructure of carbon nanotubes remains a hurdle for their widespread use. Poly(N-cyclopropylacrylamide), containing 5 mol % pyrene-bearing repeat units (p-PNCPA), is shown to vary the dispersion state of single-walled carbon nanotubes (SWNTs) in water. This is a thermo-responsive polymer whose conformation changes with temperature, which in turn leads to changes in the nanotube dispersion state. Cryo-TEM micrographs show that SWNTs stabilized using p-PNCPA transitions from a more exfoliated to a more bundled state as the aqueous suspension temperature is raised above the lower critical solution temperature (LCST) of the polymer (approximately 30 degrees C). Viscosity measurements on SWNT/p-PNCPA aqueous suspensions show shear thinning and near Newtonian behavior at 10 and 50 degrees C, respectively. Drying of these suspensions produces composites whose microstructure and electrical conductivity vary with drying temperature. This behavior has significant implications for the processing of carbon nanotubes and tailoring of composite properties. Such stimuli-controlled dispersion of carbon nanotubes could have a variety of applications in nanoelectronics, sensing, and drug and gene delivery systems.

The exfoliation of SWCNT bundles examined by simultaneous Raman scattering and photoluminescence spectroscopy

Simultaneous measurements of Raman scattering and photoluminescence (PL) prove to be a powerful method for quantifying the bundling states of single-walled carbon nanotube (SWCNT). This paper presents physical analysis and experimental evidence to establish that the G-band normalized photoluminescence, which is determined from the simultaneously acquired Raman scattering and PL emission spectra, can serve as a good indicator for quantifying the degree of exfoliation of SWCNT dispersions. Without introducing the complications of sampling geometry and instrumental correction, this indicator directly relates to the intrinsic physical properties of a given SWCNT sample, namely, the absorption cross-section, differential Raman scattering cross-section, and PL quantum yield of SWCNT. An inverse linear relationship between the G-band normalized 267 cm −1 RBM intensity and the PL emission intensities for SWCNT dispersions with different degrees of exfoliation was experimentally observed, indicating this can be used for quantitative characterization of the degree of exfoliation for a given SWCNT sample. An in-depth analysis of the indicators of the degree of exfoliation in various as-sonicated SWCNT dispersions highlights a two-stage exfoliation mechanism of SWCNT bundles under sonication.

Two-Color Sum-Frequency Generation Study of Single-Walled Carbon Nanotubes on Silver

We present the first measurement of the interfacial vibrational and electronic states of single-walled carbon nanotubes (SWCNTs) on a silver substrate by using two-color IR−visible sum frequency ge...

Thermophysical properties of carbon nanotubes in toluene under high pressure

It investigates the thermophysical properties and the equation of state of single-walled carbon nanotubes in liquid toluene. The equation of state is established for the investigated system. It analyzes the dependence of its constants on temperature and characteristics of the structure. Numerical values of isothermal elastic modulus are obtained as functions of pressure. It is found that the addition of carbon nanotubes in toluene caused the increase of the energy of molecular interaction in the investigated system.

Femtosecond Two-Pulse Cross Correlation in Single-Walled Carbon Nanotubes: Evedence of Intersubband Transitions and Role of G-Band-Assisted Transitions

By femtosecond pump-probe absorption spectroscopy we demonstrated intersubband and phonon-assisted transitions in single-walled carbon nanotubes (SWNTs). After resonant excitations of specific tube types, electrons are promoted from the ground state to the second excitonic state (EX2) in PLV-prepared SWNTs or to vibrational excited levels of the first excitonic state (EX1) in HiPco-prepared SWNTs. At time-zero delay a [1 + 1] resonant cross-correlation due to short lifetimes of higher excited states or phonon modes can be observed. These results suggest that intersubband transitions between the excitonic states in SWNTs are allowed. The interpretation of the temporal profiles at pump/probe combinations implies that the G-band mode, which is known to have a high Raman intensity, plays an important role for the interpretation of both femtosecond cross-correlation measurements and photoluminescence spectra.

Changes in the Electronic States of Single-Walled Carbon Nanotubes as Followed by a Raman Spectroelectrochemical Analysis of the Radial Breathing Mode

A detailed in situ Raman spectroelectrochemical analysis of the radial breathing mode (RBM) of single-walled carbon nanotubes (SWCNTs) in bundles is presented. The intensity of the RBM feature exhibits striking changes even before the electrode potential reaches the level of the Van Hove singularity, which is involved in optical transitions causing a resonance enhancement of the Raman scattering. A difference in the behavior of semiconducting and metallic tubes has been found to reflect the different electronic structure of these two types of SWCNTs. The intensity of the RBM of metallic tubes starts to change immediately with the electrode potential which is scanned from the open circuit value to a distinct electrode potential. On the other hand, semiconducting tubes give a significant change of the spectral intensity at potentials deviating by ca. ±0.5 V from the open circuit value. The resonant Raman spectra of doped carbon nanotubes are shown to reflect not only the filling of the particular electronic states by electrons or holes but also some more fundamental changes in their energies and densities.

The quantitative characterization of the dispersion state of single-walled carbon nanotubes using Raman spectroscopy and atomic force microscopy

A quantitative method to evaluate the degree of dispersion of single-walled carbon nanotubes (SWCNTs), produced by the high-pressure carbon monoxide process, was developed using Raman spectroscopy and atomic force microscopy (AFM). Nanotubes were dispersed in sodium dodecyl sulfate aqueous solution at seven different dispersion states by controlling ultra-sonication and centrifugation parameters. It is known that the intensity of a Raman peak at 267 cm −1, at the excitation wavelength of 785 nm, is qualitatively proportional to the degree of aggregation. Here, we provide a quantitative calibration technique which involves single-layer spin-deposition of SWCNTs on mica substrates and z-scan analysis of AFM. The trend of the height measurements of AFM precisely matched that of the Raman peak at 267 cm −1. Therefore, this approach can be used to quantitatively characterize the dispersion state of SWCNTs.

Excited Excitonic States in Single-Walled Carbon Nanotubes

Polarized photoluminescence excitation spectroscopy on individual SWNTs reveals not only the longitudinal and transverse E 11, E 22, and E 12 ground-state excitons but also excited excitonic states including the continuum. When heated, SWNTs are known to undergo a bandgap shift transition (BST), which effectively changes the nanotube dielectric environment. Here, we show that the entire spectrum of excitonic resonances blue shifts under this transition, with excited states showing larger shifts, approaching 100 meV for a 1 nm diameter nanotube. The excitonic binding energy, Coulomb self-energy correction, and dielectric shift under the BST are estimated. Analysis of this blue shift reveals the dominant effect of dielectric screening on SWNT excitonic states.

Triplet State Absorption in Carbon Nanotubes: A TD−DFT Study

We predict properties of triplet excited states in single-walled carbon nanotubes (CNTs) using a time-dependent density-functional theory (TD-DFT). We show that the lowest triplet state energy in CNTs to be about 0.2-0.3 eV lower than the lowest singlet state energies. Like in pi-conjugated polymers, the lowest CNT triplets are spatially localized. These states show strong optical absorption at about 0.5-0.6 eV to the higher lying delocalized triplet states. These results demonstrate striking similarity of the electronic features between CNTs and pi-conjugated polymers and provide explicit guidelines for spectroscopic detection of CNT triplet states.

Ultrafast exciton dynamics in semiconducting single-walled carbon nanotubes¶

To define the origin of the electronic excited states of single-walled carbon nanotubes, femtosecond fluorescence up-conversion and frequency-resolved transient absorption measurements were used for selective excitation/probing of particular nanotubes in a micelle-dispersed preparation enriched with individual nanotubes. Analysis of the temporal and intensity profiles of the transient signals enable one to unambiguously identify the excitonic nature of elementary excitations in semiconducting nanotubes, and the predominant dynamical process as a remarkable manifestation of exciton–exciton annihilation. These studies convincingly support the conclusion that the longest wavelength electronic excitations are of excitonic origin. In addition, evidence is shown for bi-exciton formation in single walled nanotubes. ¶We dedicate this paper to our friend and colleague Bob Harris, whose insights and capacity for wonder are a source of inspiration.

Long-Lived Charged States in Single-Walled Carbon Nanotubes

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.

Exciton binding energies in carbon nanotubes from two-photon photoluminescence

One- and two-photon luminescence excitation spectroscopy showed a series of distinct excitonic states in single-walled carbon nanotubes. The energy splitting between one- and two-photon-active exciton states of different wavefunction symmetry is the fingerprint of excitonic interactions in carbon nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are supported by ab-initio calculations of the linear and non-linear optical spectra, prove that the elementary optical excitations of carbon nanotubes are strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole pairs, stable even at room temperature. This alters our microscopic understanding of both the electronic structure and the Coulomb interactions in carbon nanotubes, and has direct impact on the optical and transport properties of novel nanotube devices.

Ground states of potassium adsorbate on single-walled carbon nanotubes

We have investigated the ground states of single-walled carbon nanotubes with potassium atoms adsorbed in the center of the hexagonal unit using the Monte-Carlo method and the first-principles approach. We predict that two types of high coverage structures ( x = 1 / 2 , 2 / 3 ) are stable for adsorption on the ( 6 , 0 ) carbon nanotube in addition to low coverage structures of adsorbate on graphite. It is shown that the curvature of carbon nanotube surface enhances the stability of high coverage structures of adsorbate.